In modern IP-based voice networks DTMF (Dual Tone Multi-Frequency) B-number dialing is still required in order to support legacy POTS (Plain Ordinary Telephone Service) subscribers. H.248 signaling is used to convey the dialed DTMF digits from access gateways to call control servers, where the number analysis and call routing is performed. Since the H.248 signaling used to convey individual dialed DTMF digits can constitute a significant proportion of the total H.248 signaling volume, digit maps are employed to collect a predefined string of dialed DTMF digits in the access gateway and to transmit the digit string as a single H.248 message towards the call control server. The main purpose of the digit map is to minimize H.248 signaling load for outgoing call attempts by analogue subscribers connected via access gateways. Digit maps allow the network to potentially support more successful outgoing call attempts.
For the reduction in H.248 signaling load to be significant, it must be possible to apply the digit maps in an efficient manner, meaning that as many digits as possible are collected via digit maps and sent to the call control server in H.248 digit strings.
In reality, utilizing digit maps in an efficient manner can be hard to achieve, since a single call control server may serve many hundreds of access gateways, each of which may serve thousands of individual subscribers. Per subscriber, different dialing plans may be applicable, meaning that many different digit maps may need to be defined and allocated accordingly. This implies a fairly significant administrative overhead in manually defining, provisioning and maintaining multiple digit maps effectively.